In a mobile communications system, cell search is a procedure by which a user equipment (UE) acquires time and frequency synchronization in a cell and detects a cell ID. The UE is time synchronized when start of symbols as well as a radio frame is found. Both symbol timing and frame timing need to be found for completing a cell search.
To improve the symbol timing performance, the synchronization signals are envisaged to be multiplexed several times in a radio frame. Thereby output statistics from a correlator performing symbol timing acquisition can be accumulated, and the probability of correct symbol timing is improved. Furthermore, to allow efficient handover between different radio systems, it is anticipated that synchronization signals are multiplexed several times in a radio frame. However, a consequence of such multiplexing is that frame timing may not follow directly from symbol timing. Mechanisms are therefore needed that, given symbol timing, frame timing can be determined.
Two classes of synchronization channel (SCH) to be used in cell search can be defined: a non-hierarchical SCH and a hierarchical SCH. A non-hierarchical SCH includes cell-specific signals that serve for timing acquisition, frequency acquisition, and cell ID detection. A hierarchical SCH includes at least two signals; a known primary cell-common signal used only for symbol timing acquisition, and a cell-specific signal used for frame timing synchronization, frequency synchronization, and cell ID detection.
Refer to a list of reference documents at the end of this specification, a previously used concept, shown in reference documents [1], [2] and [3], comprises transmission of different signals in SCH slots within a frame. Given symbol timing, signals in the SCH slots are detected independently, but together they represent elements of a codeword from a synchronization code. Since SCH is periodically transmitted, the receiver can detect any cyclic version of a codeword. The synchronization code is therefore constructed such that all cyclic shifts of a codeword are unique and no codeword is a cyclic shift of another codeword. Thereby frame timing can be uniquely determined from a cyclic shift of the detected codeword.
In a fully non-hierarchical SCH, it is foreseen that both cell ID and frame synchronization are detected only from SCH signals within a frame, i.e. no hierarchical cell ID grouping or other channels should be needed. Correspondingly, in a non-hierarchical solution, a user equipment (UE) would need to decode and compute metrics for 512 codewords (cell IDs) and their cyclic shifts at once. Since cell ID detection is done both initially for finding a home cell and continuously for supporting mobility by finding neighbor cells, such an exhaustive procedure may become overly tedious, and consume a lot of computing and power resources in the UE and prolong the cell search time. Moreover, as has been discussed for a E-UTRA system, not only cell IDs but also additional cell-specific information may be included in the cell search procedure, e.g. channel bandwidth, number of antennas, cyclic prefix lengths etc. This would require even larger sets of codewords that need to be efficiently decoded.
It is desirable to give codewords some form of structure that can be utilized by the receiver.